Selecting only the bottle's INSIDE bottom face, to form upward creating a 'hill' of glass in the bottom
Blender allows you to hide vertices for convenience in Edit Mode

Apply the Solidify modifier.

In edit mode select the vertices you do not want to edit.
Hide those vertices.  Edit the visible vertices.



Alternative.
Change to wireframe mode.  Edit mode. Select the top of the hill vertex which you can see with difficulty.  Press Control [Numeric Keypad + ] to grow your selection.  Then hide the unselected.

[Control I] can invert your selection.